CPR (Cardiopulmonary Resuscitation) feedback systems have recently gained attention as a method for improving the quality of CPR on a cardiac arrest victim. One typical feature of such systems is to measure the compression depth and rate during chest compressions, compare these with accepted guideline limits, and give verbal or visual feedback to the rescuer. For instance, the CPR feedback system could provide feedback when the compression depth does not meet the accepted value of 3.8-5.1 cm.
A system for giving feedback on compressions typically consists of a sensor pad to be placed on the victim's chest. The sensor pad may contain an accelerometer and optionally a force sensor. The compression depth measurement is usually based on double integration of acceleration. However, if not all zero offset is removed from the acceleration signal prior to double integration, the integration is likely to “run off” and the estimated depth will not be useable for giving feedback.
One approach that has been used to remove zero offset from the acceleration signal before double integration to obtain chest compression depth is disclosed in U.S. Pat. No. 6,306,107 to Myklebust et al. The method includes resetting the depth and velocity to zero each time a force switch is activated at the onset of a new compression.
Another approach described in “Compression Depth Estimation for CPR Quality Assessment Using DSP on Accelerometer Signals,” IEEE Transactions on Biomechanical Engineering, Vol. 49, No. 3, March 2002, Aase et al., describes a method where the offset in acceleration is removed after each compression, by setting the boundary conditions so that the chest is assumed to return to the same position and speed when the force is released. The integration limits determining the boundary conditions are determined by the help of a force switch. The disadvantage of this method is that the method does not provide a real-time, sample-by-sample assessment of depth, but only calculates depth of the previous compression.
A more recent technique, described in U.S. Pat. No. 7,118,542 to Palazzolo et al., incorporated herein by reference in its entirety, describes a method of filtering and integrating acceleration to obtain depth. A moving average of past starting points is used to estimate the starting point of each compression. Additionally, an independent reference signal, such as an ECG (Electrocardiogram) signal with compression artifacts, may be used to assist in determining the starting points. Various types of noise reference signals may be used to estimate and remove sources of noise in the acceleration signal prior to integration, by correlating the noise reference signals with the acceleration signal.
One disadvantage with the method described in U.S. Pat. No. 7,118,542 is that it does not compensate the acceleration or depth signals for the distortion caused by the filters. In general, filters not only remove frequencies outside their pass-band, but also attenuates or delays certain frequency components within the pass-band. This may cause distortion of the filtered signal relative to the original signal.
For instance, if a high-pass filter is used to remove drift in an acceleration offset, the filter will also distort frequencies in the vicinity of its cut-off frequency. Upon double integration, these distortions will also cause distortion of the resulting depth signal. If the cut-off frequency of the filter is too close to the compression frequency, the depth signal may be significantly distorted. On the other hand, the lower the cut-off frequency, the less effective the filter will be in removing drift.
Therefore, there is a need to reduce the effects of filter distortion on the compression depth signal, while maintaining an adequate removal of sensor drift.